Getting a bearing on the origins of Zn

I was struck by Twain Braden’s recent comment in Chartroom Chatter (Celestial tool and star chart Issue 124, Sept./Oct. 2002) that most people these days barely know what azimuth is, not to mention its obscure abbreviation, “Zn.” I was curious enough about the remark to see if I could track the symbol to its source in standard works of l9th- and 20th-century nautical astronomy. The concept of Zn is key, of course, to any understanding of the central problem of celestial navigation: the line of position.

The simplest definition of azimuth is the bearing of a heavenly body from a fixed point on land or at sea. To be more precise, we might say azimuth is the bearing to the body’s geographical position (the celestial coordinates of declination and hour angle reformulated as latitude and longitude). Mathematically, the bearing is measured the shortest way from the poles, east or west of an observer’s meridian to a maximum of l80°. This means that to obtain true azimuth on a 360° compass rose (Zn), an adjustment has to be made. There is a single exception. In the Northern Hemisphere, if the body lies east, the mathematical bearing is labeled “Z” and, in this case, Z is the same as Zn. But if the body lies west, Z is converted to Zn by subtracting it from 360. In the Southern Hemisphere the benchmark reference is l80°, not 360. For a body lying east of the meridian, Zn is l80 minus Z; west, l80 plus Z.

Capt. Lecky in Wrinkles in Practical Navigation (1891 edition) has a chapter on Sumner lines (lines of position) but makes no mention of the symbols Z or Zn. Norie’s Epitome of Practical Navigation (1917 edition) employs the symbol Az to describe true azimuth on a 360° scale. Bowditch in l9l8 confused the issue further by using Z. Not until the l930s and early l940s was there widespread use of Zn in inspection tables and texts, and only with George W. Mixter in his popular Primer of Navigation (1944 edition) do we pinpoint what appears to be the origin of the Zn abbreviation.

Navy teaching, writes Mixter, is to convert Z to Zn, which is azimuth as measured from the north point clockwise around through to 360°, as most convenient for plotting lines of position and for compass work. The “n” in Zn, according to Mixter, stands for nothing more than north. Zn, he says, means azimuth measured from north. For azimuth itself, Webster’s Second gives this as the root: as-sumut, an Arabic word meaning way, or possibly a point on the horizon.

Alan Littell is a freelance journalist and celestial navigation fan who lives in Alfred, N.Y., and Athens, Greece.

By Ocean Navigator